The dye 1-ethyl-2-(3-(1-ethylnaphtho(1,2d)thiazolin-2-ylidene)-2-methylpropenyl-naphtho( 1,2d)thiazolium bromide is a metachromatic dye of unusual sensitivity for differentiating various classes of macromolecules by color. Methods for staining proteins, phosphoproteins, sialoglycoproteins, glycosaminoglycans and nucleic acids differentially in tissue sections and on polyacrylamide gels following electrophoresis were developed. These methods were applied to the study of milk and breast tissue proteins. A histochemical study of normal and neoplastic human breast tissue was done and the macromolecules listed above were identified.